In a semiconductor field or the like, when a round film is framed on a disk-shaped substrate, the variation in thickness of the film is required to be 0.1 to 1.0 μm. To obtain the variation in thickness of the film within the intended range, the film formation is performed by spin coating. The spin coating includes: fixing a disk-shaped substrate on a circular rotary stage; applying a necessary amount of material to the center of the substrate; spinning the rotary stage at high speed to spread the material on the rotary stage over the entire surface of the substrate by centrifugal force, thus foaming a film.
The material scatters during the spin coating, and the material use efficiency of the spin coating is as low as about 30%. Moreover, a cover is provided around the rotary stage to prevent the apparatus from being contaminated by the scattered material. The cover needs to be regularly cleaned, and the conditions for the application are set for each cleaning work. This requires a certain amount of maintenance time, thus reducing the operation rate of the film forming system.
As a means for improving the problems of the spin coating, there is a spiral coating method. The spiral coating method includes: fixing a disk-shaped substrate to the circular rotary stage; spinning the rotary stage with a gap between the outlet surface of the application nozzle and the surface of the substrate being kept at a predetermined value; causing the material to be discharged from the application nozzle with a metering pump, which is capable of controlling the flow rate, while linearly moving the application nozzle from the center of the substrate to the periphery to form a spiral application trajectory, thus forming a film on the entire surface of the substrate.
In the aforementioned spiral coating, the material scatters less than that in the spin coating and is therefore effective on an improvement in the material use efficiency. Furthermore, the spiral coating does not need the cover for preventing the material from scattering. In the spiral coating, the regular cleaning of the cover, which is necessary for the spin coating, is eliminated, thus shortening the maintenance time. The spiral coating is therefore effective on an increase in the operating rate. However, in this method, the application forms the spiral trajectory, and the application nozzle brought close to the substrate moves along the spiral trajectory. The application nozzle is therefore more susceptible to contamination than the spin coating, and the contamination increases the variation in film thickness, thus resulting in low uniformity of the film thickness.